The present invention is to improve abrasion resistance of rolling bearings to be incorporated in rotation supporters of jet engines for aircraft or gear boxes and to be served under circumstance at high temperatures, thereby to heighten endurance of rolling bearings.
In rotation supporters of jet engines for aircraft or gear boxes, rolling bearings 1a to 1c as shown in FIGS. 1A to 1C are incorporated as bearings for aircraft. Three kinds of rolling bearings 1a to 1c are shown in FIGS. 1A to 1C. The rolling bearing 1a of FIG. 1A is a ball bearing of single row deep groove type with four point-contact points in which two-piece housing inner ring 2a is provided. The rolling bearing 1b of FIG. 1B is an angular ball bearing having an inner ring 2b of counter bored type. The bearing of FIG. 1C is a single row cylindrical roller bearing.
In each of these cases, the rolling bearings 1a to 1c have inner rings 2a to 2c, outer rings 5a to 5c, a plurality rolling elements 6a to 6c rotatably disposed therebetween, and retainers 7a to 7c, respectively. The inner rings 2a to 2c are made of iron based alloy and respectively have inner ring raceways 3a to 3c in outer circumferential surfaces thereof. The outer rings 5a to 5c are made of iron-alloy and respectively have outer ring raceways 4a to 4c in inner circumferential surfaces thereof. The plurality rolling elements 6a to 6c are rotatably disposed between the inner ring raceways 3a to 3c and the outer ring raceways 4a to 4c, respectively. The retainers 7a to 7c rotatably hold the rolling elements 6a to 6c, respectively. The outer circumferential surface of each of the retainers 7a to 7c is prevented from displacement in an radial direction thereof by guiding the outer circumferential surface of respective retainer 7a to 7c by means of one part of the inner circumferential surface of the outer ring 5a to 5c which (one part) is disposed outside of the outer ring raceway 4a to 4c. 
For using the rolling bearings 1a to 1c of the structures as shown in FIGS. 1A to 1C in applications particularly requiring large corrosion resistance in bearings for aircraft, there are many cases manufacturing the inner rings 2a to 2c and the outer rings 5a to 5c with SUS440C (the iron-alloy containing C 1.05 wt %, P 0.040 wt % or lower, S 0.030 wt % or lower, Mn 1.00 wt %, Si 1.00 wt %, Cr 17.00 wt %, Mo 0.75 wt %, the balance being Fe and unavoidable impurities).
Using SUS440C under circumstances at high temperatures as jet engine for aircraft or gear boxes, it did not always have enough corrosion resistance and so was disadvantageous in view of securing endurance. That is, since the inner rings 2a to 2c and outer rings 5a to 5c constituting the rolling bearings 1a to 1c to be applied to usage as above mentioned are tempered at high temperatures in the course of the production, the hardness in the surfaces including the inner ring raceways 3a to 3c and the outer ring raceways 4a to 4c is around Hv650 at the most. This value is poor in the corrosion resistance comparing with the hardness Hv being about 700 of heat resistant bearing steels such as M50 (the iron-alloy containing C: 0.80 wt %, P: 0.030 wt % or lower, S: 0.030 wt % or lower, Mn: 0.30 wt %, Si: 0.25 wt %, Cr: 4.00 wt %, V: 1.00 wt %, Mo: 4.25 wt %, the balance being Fe and unavoidable impurities). On the other hand, the heat resistant bearing steels as M50 are difficult to secure a sufficient endurance in term of corrosion resistance.
In view of such circumstances, there are cases of using heat resistant and corrosion resistant steels such as Pyrowear 675 (trade name=iron-alloy containing C: 0.07 wt %, P: 0.005 wt % or lower, S: 0.003 wt % or lower, Mn: 0.65 wt %, Si: 0.40 wt %, Cr: 13.00 wt %, V: 0.60 wt %, Mo: 1.80 wt %, Ni: 2.60 wt %, Co: 5.40 wt %, the balance being Fe and unavoidable impurities). However, if fabricating the inner rings 2a to 2c and the outer rings 5a to 5c of the rolling bearings 1a to 1c with such heat resistant and corrosion resistant steels, being accompanied with carburizing treatments for hardening surfaces, Cr carbides are precipitated in the surfaces of the inner rings 2a to 2c and the outer rings 5a to 5c, so that the corrosion resistance in the surface is lowered and it is still difficult to secure the enough endurance.
Further, in case of using the rolling bearings 1a to 1c as shown in FIGS. 1A to 1C for supporting such as turbines of jet engines for aircraft, parts guiding the outer circumferential surfaces of the retainers 7a to 7c often causes abrasion or damage in parts of the inner circumferential surfaces of the outer rings 5a to 5c. For avoiding such accidents, the conventional technique carries out a treatment for forming a hard film such as TiN in the guiding part of the outer circumferential surfaces. But since this hard film treatment is very expensive, and not only it invites cost-up of the rolling bearings 1a to 1c, but also the hard film often comes off, it does not always bring about an effect of improving a secure abrasion resistibility.
Besides, with respect to the rolling bearings 1a to 1c to be used to jet engines for aircraft or gear boxes, in a case of a so-called hybrid bearing where the inner rings 2a to 2c as well as the outer rings 5a to 5c are made of bearing steels, but the rolling elements are made of ceramic in order to heighten a seizure resistance, the improvement of the abrasion resistibility in the inner rings 2a to 2c and the outer rings 5a to 5c become a furthermore problem. That is, in the hybrid bearing, since the contacting condition between the inner rings 2a to 2c and the rolling elements 6a to 6c and/or the contacting condition between the outer rings 5a to 5c and the rolling elements 6a to 6c takes place between materials of heterogeneous kinds, the seizure resistance of the rolling elements 6a to 6c can be improved in comparison with the bearing having the rolling elements made of bearing steels (as well as the inner and outer rings). However, the hardness of the bearing steel comprising the inner rings 2a to 2c and the outer rings 5a to 5c is far lower than that of the ceramic composing the rolling elements 6a to 6c, so that when foreign matters as dust or dirt go into the bearings 1a to 1c, the inner raceways 3a to 3c and the outer raceways 4a to 4c are considerably worn.
In view of these situations, the invention has been made to realize such rolling bearings at low cost, having excellent abrasion resistance, and good corrosion resistance, if needed.
In addition, the invention has also been made to realize a method of making such rolling bearings.
The above-mentioned object can be achieved by a rolling bearing according to a first aspect of the invention comprising an inner ring, an outer ring, a plurality of rolling elements rotatably disposed therebetween, and a retainer. The inner ring is made of iron-alloy and has an inner ring raceway in an outer circumferential surface thereof. The outer ring is made of iron-alloy and has an outer ring raceway in an inner circumferential surface thereof. The plurality of rolling elements are rotatably disposed between the inner ring raceway and the outer ring raceway. The retainer has the inner circumferential surface and the outer circumferential surface and rotatably holds each of the rolling elements. Either of the inner and outer circumferential surfaces of the retainer is guided by a part of either of the outer circumferential surface of the inner ring and the inner circumferential surface of the outer ring for preventing the displacement in the radial direction of the retainer.
In the rolling bearing of the invention, a fine nitride layer comprising a compound layer and a diffusion hardened layer and has hardness of Hv900 or higher is formed in a part which guides the circumferential surface of a retainer by means of at least one part of any surface of the inner ring raceway surface and the outer ring raceway surface as well as the outer circumferential surface of the inner ring and the inner circumferential surface of the outer ring.
The above-mentioned object can also be achieved by a method of making the above-mentioned rolling bearing, according to a second aspect of the present invention, comprising the steps of:
performing a low-temperature nitriding treatment at 300 to 400xc2x0 C. so as to form nitride layer in surfaces of the inner ring and the outer ring of the rolling bearing.
As the low-temperature nitriding treatment, an NV nitriding treatment being xe2x80x9cNV-300xe2x80x9d of Daido Hokusan K.K. may be desirably used.